The present invention relates to an engine air-fuel ratio control unit which is designed to determine a basic fuel amount for controlling the engine to a prescribed air-fuel ratio by arithmetic operations performed on the basis of the information of the intake air amount of the engine and also to correct the signal of the amount of fuel to be fed into the injector, in such a manner as to operate the engine at a theoretical air-fuel ratio, on the basis of the output information from O.sub.2 sensors installed respectively on the left and right exhaust banks.
In general, the system to which this type of engine air-fuel ratio control unit is applied, is constructed as illustrated in FIG. 3, in which reference number 1 indicates an engine, reference number 2 indicates an air flow sensor, reference number 3 indicates a throttle valve, reference number 8 indicates an air-fuel ratio control unit, and reference number 9 indicates a revolution sensor which detects the revolutions of the engine 1. Moreover, as the exhaust system is divided between the two banks, namely, the left bank and the right bank, O.sub.2 sensors and component parts mentioned in the following are provided each on the left side and the right side. That is, reference number 4 indicates an O.sub.2 sensor (right), which performs the detection of the exhaust gas, and reference number 5 indicates an O.sub.2 sensor (left), which similarly performs the detection of the exhaust gas. Reference number 6 indicates an injector (right), which performs the injection of the fuel, and reference number 7 indicates an injector (left), which similarly performs the injection of the fuel. Reference number 10 indicates a ternary catalytic converter (right), and reference number 11 indicates a ternary catalytic converter (left).
Moreover, FIG. 4 shows a detailed block construction of the air-fuel ratio control unit 8 shown in the construction drawing of the engine control system in FIG. 3. In FIG. 4, reference number 20 indicates a basic fuel amount calculating means for calculating the basic fuel amount on the basis of the detected amount of an intake air, reference numbers 21 and 22 indicate an A/F feedback correction means, which makes corrections of the air-fuel ratio feedback on the basis of the detected output information from the O.sub.2 sensors, and reference number 23 indicates an A/F feedback determining means, which performs control by determining whether the basic fuel amount is to be fed into the injector (right) 6 and the injector (left) 7, respectively, or whether a corrected amount of the fuel as determined by the two systems of the air-fuel feedback correction means 21 and 22 is to be fed into the injectors.
FIG. 5 shows a timing chart illustrating the relationship between the output information from these O.sub.2 sensors and the output time widths of the injectors 6 and 7, which are installed respectively on the left bank and the right bank, in case the individual O.sub.2 sensors are in their normal state. That is to say, FIG. 5(a) shows the waveform of the output from the O.sub.2 sensor (right) 4, and FIG. 5(b) shows the time duration of fuel injection from the injector (right) 6, which corresponds to the above waveform. As shown in these charts, the air-fuel feedback correction means 21 makes a correction in such a manner as to reduce the amount of the fuel fed, when the signal from the O.sub.2 sensor (right) 4 increases and rises above the threshold value voltage V.sub.1, which corresponds to the theoretical air-fuel ratio, and, as the result of this correction, the time T for fuel injection (right) from the injector (right) 6 is shortened. Also, when the output from the O.sub.2 sensor (right) 4 decreases and falls down below the threshold value voltage V.sub.1, the air-fuel feedback correction means 21 makes a correction in such a manner as to increase the amount of the fuel, and, as the result of this correction, the time T for fuel injection (right) from the injector (right) 6 is extended.
In reflection of these results, the waveform of the time T for fuel injection (right) will be such a waveform in amplitude fluctuating upward and downward with respect to the mean value T (right) (central value: the duration of time corresponding to the theoretical air-fuel ratio). Then, the deviations of this amount of feedback correction from the mean value T (right) are constantly renewed and stored in a memory (learning function), and, when the O.sub.2 sensor (right) 4 becomes in any abnormal state, the feedback correction is made on the basis of the corrected value (learned value) thus stored in the memory.
Also, the timing relationship between the waveform of the output from the O.sub.2 sensor (left) illustrated in FIG. 5 (c) and the time for fuel injection (left) from the injector (left) 7 illustrated in FIG. 5(d) shows a transition similar to what is described above.
Generally, the ternary catalytic converters will attain the maximum efficiency in their purification of exhaust gas when the air-fuel ratio A/F is 14.7 (the theoretical air-fuel ratio), and their purifying efficiency will be kept at a favorable level by the O.sub.2 storage effect if control is performed on the correction of the fuel amount by increasing and decreasing it in a prescribed cycle with reference to the line of the value 14.7 of the air-fuel ratio. On the contrary, the purifying efficiency will become extremely low in case the air-fuel ratio deviates from the proximity of the value 14.7 of the air-fuel ratio or in case control is not performed on the correction of the fuel amount by having it fluctuate upward and downward in relation to the line of the value 14.7 of the air-fuel ratio. In case one of the O.sub.2 sensors has a failure, the conventional air-fuel control unit for engine corrects the amount of the fuel for the bank where the failure has occurred by arithmetic operations performed on the basis of the value learned at the time when the failing O.sub.2 sensor was in a normal state, and consequently the corrected value will be a certain fixed value. As the result, the conventional unit presents the problem that it is not capable of correcting the amount of the fuel by moving it upward and downward in a prescribed cycle in relation to the line of the value 14.7 of the air-fuel ratio and consequently that it is incapable of effectively purifying the exhaust gas. Additionally, in case a deviation or the like has occurred in the learned value, the conventional unit fails to make any sufficient correction of the amount of the fuel, so that the ternary catalytic converters cannot be utilized effectively.